428 research outputs found
Improving sustainability through intelligent cargo and adaptive decision making
In the current society, logistics is faced with the challenge to meet more stringent sustainability goals. Shippers and transport service providers both aim to reduce the carbon footprint of their logistic operations. To do so, optimal use of logistics resources and physical infrastructure should be aimed for. An adaptive decision making process for the selection of a specific transport modality, transport provider and timeslot (aimed at minimisation of the carbon footprint) enables shippers to achieve this. This requires shippers to have access to up-to-date capacity information from transport providers (e.g. current and scheduled loading status of the various transport means and information on carbon footprint) and traffic information (e.g. city logistics and current traffic information). A prerequisite is an adequate infrastructure for collaboration and open exchange of information between the various stakeholders in the logistics value chain to obtain the up-to-date information. This paper gives a view on how such an advanced information infrastructure can be realised, currently being developed within the EU iCargo project. The paper describes a reference logistics value chain, including business benefits for each of the roles in the logistics value chain of aiming for sustainability. A case analysis is presented that reflects a practical situation in which the various roles collaborate and exchange information for realizing sustainability goals, using adaptive decision making for selecting a transport modality, transport provider, and timeslot. A high-level overview is provided of the requirements on and technical implementation of the supporting advanced infrastructure for collaboration and open information exchange.In the current society, logistics is faced with the challenge to meet more stringent sustainability goals. Shippers and transport service providers both aim to reduce the carbon footprint of their logistic operations. To do so, optimal use of logistics resources and physical infrastructure should be aimed for. An adaptive decision making process for the selection of a specific transport modality, transport provider and timeslot (aimed at minimisation of the carbon footprint) enables shippers to achieve this. This requires shippers to have access to up-to-date capacity information from transport providers (e.g. current and scheduled loading status of the various transport means and information on carbon footprint) and traffic information (e.g. city logistics and current traffic information). A prerequisite is an adequate infrastructure for collaboration and open exchange of information between the various stakeholders in the logistics value chain to obtain the up-to-date information. This paper gives a view on how such an advanced information infrastructure can be realised, currently being developed within the EU iCargo project. The paper describes a reference logistics value chain, including business benefits for each of the roles in the logistics value chain of aiming for sustainability. A case analysis is presented that reflects a practical situation in which the various roles collaborate and exchange information for realizing sustainability goals, using adaptive decision making for selecting a transport modality, transport provider, and timeslot. A high-level overview is provided of the requirements on and technical implementation of the supporting advanced infrastructure for collaboration and open information exchange.In the current society, logistics is faced with the challenge to meet more stringent sustainability goals. Shippers and transport service providers both aim to reduce the carbon footprint of their logistic operations. To do so, optimal use of logistics resources and physical infrastructure should be aimed for. An adaptive decision making process for the selection of a specific transport modality, transport provider and timeslot (aimed at minimisation of the carbon footprint) enables shippers to achieve this. This requires shippers to have access to up-to-date capacity information from transport providers (e.g. current and scheduled loading status of the various transport means and information on carbon footprint) and traffic information (e.g. city logistics and current traffic information). A prerequisite is an adequate infrastructure for collaboration and open exchange of information between the various stakeholders in the logistics value chain to obtain the up-to-date information. This paper gives a view on how such an advanced information infrastructure can be realised, currently being developed within the EU iCargo project. The paper describes a reference logistics value chain, including business benefits for each of the roles in the logistics value chain of aiming for sustainability. A case analysis is presented that reflects a practical situation in which the various roles collaborate and exchange information for realizing sustainability goals, using adaptive decision making for selecting a transport modality, transport provider, and timeslot. A high-level overview is provided of the requirements on and technical implementation of the supporting advanced infrastructure for collaboration and open information exchange
XMMU J174716.1-281048: a "quasi-persistent" very faint X-ray transient?
The X-ray transient XMMU J174716.1-281048 was serendipitously discovered with
XMM-Newton in 2003. It lies about 0.9 degrees off the Galactic Centre and its
spectrum shows a high absorption (~8 x 10E22 cm^(-2)). Previous X-ray
observations of the source field performed in 2000 and 2001 did not detect the
source, indicative of a quiescent emission at least two orders of magnitude
fainter. The low luminosity during the outburst (~5 x 10E34 erg/s at 8 kpc)
indicates that the source is a member of the ``very faint X-ray transients''
class. On 2005 March 22nd the INTEGRAL satellite caught a possible type-I X-ray
burst from the new INTEGRAL source IGR J17464-2811, classified as fast X-ray
transient. This source was soon found to be positionally coincident, within the
uncertainties, with XMMU J174716.1-281048. Here we report data analysis of the
X-ray burst observed with the IBIS and JEM-X telescopes and confirm the type-I
burst nature. We also re-analysed XMM-Newton and Chandra archival observations
of the source field. We discuss the implications of these new findings,
particularly related to the source distance as well as the source
classification.Comment: 4 pages, 8 figures, accepted for publication in A&A Letter
On the Origin of the Absorption Features in SS433
We present high-resolution optical spectroscopy of the X-ray binary system
SS433, obtained over a wide range of orbital phases. The spectra display
numerous weak absorption features, and include the clearest example seen to
date of those features, resembling a mid-A type supergiant spectrum, that have
previously been associated with the mass donor star. However, the new data
preclude the hypothesis that these features originate solely within the
photosphere of the putative mass donor, indicating that there may be more than
one region within the system producing an A supergiant-like spectrum, probably
an accretion disc wind. Indeed, whilst we cannot confirm the possibility that
the companion star is visible at certain phase combinations, it is possible
that all supergiant-like features observed thus far are produced solely in a
wind. We conclude that great care must be taken when interpreting the behaviour
of these weak features.Comment: Accepted for publication in MNRAS, 8 pages, 6 figure
X-Ray Spectroscopy of the Low-Mass X-ray Binaries 2S 0918-549 and 4U1543-624: Evidence for Neon-Rich Degenerate Donors
We present high-resolution spectroscopy of the neutron-star/low-mass X-ray
binaries 2S 0918-549 and 4U 1543-624 with the High Energy Transmission Grating
Spectrometer onboard the Chandra X-ray Observatory and the Reflection Grating
Spectrometer onboard XMM-Newton. Previous low-resolution spectra of both
sources showed a broad line-like feature at 0.7 keV that was originally
attributed to unresolved line emission. We recently showed that this feature
could also be due to excess neutral Ne absorption, and this is confirmed by the
new high-resolution Chandra spectra. The Chandra spectra are each well fit by
an absorbed power-law + blackbody model with a modified Ne/O number ratio of
0.52+/-0.12 for 2S 0918-549 and 1.5+/-0.3 for 4U 1543-624, compared to the
interstellar-medium value of 0.18. The XMM spectrum of 2S 0918-549 is best fit
by an absorbed power-law model with a Ne/O number ratio of 0.46+/-0.03,
consistent with the Chandra result. On the other hand, the XMM spectrum of 4U
1543-624 is softer and less luminous than the Chandra spectrum and has a
best-fit Ne/O number ratio of 0.54+/-0.03. The difference between the measured
abundances and the expected interstellar ratio, as well as the variation of the
column densities of O and Ne in 4U 1543-624, supports the suggestion that there
is absorption local to these binaries. We propose that the variations in the O
and Ne column densities of 4U 1543-624 are caused by changes in the ionization
structure of the local absorbing material. It is important to understand the
effect of ionization on the measured absorption columns before the abundance of
the local material can be determined. This work supports our earlier suggestion
that 2S 0918-549 and 4U 1543-624 are ultracompact binaries with Ne-rich
companions.Comment: 11 pages, 5 figures, major revisions including addition of XMM
spectral analysis, accepted for publication in the Astrophysical Journal,
vol. 59
Unstable Nonradial Oscillations on Helium Burning Neutron Stars
Material accreted onto a neutron star can stably burn in steady state only
when the accretion rate is high (typically super-Eddington) or if a large flux
from the neutron star crust permeates the outer atmosphere. For such situations
we have analyzed the stability of nonradial oscillations, finding one unstable
mode for pure helium accretion. This is a shallow surface wave which resides in
the helium atmosphere above the heavier ashes of the ocean. It is excited by
the increase in the nuclear reaction rate during the oscillations, and it grows
on the timescale of a second. For a slowly rotating star, this mode has a
frequency of approximately 20-30 Hz (for l=1), and we calculate the full
spectrum that a rapidly rotating (>>30 Hz) neutron star would support. The
short period X-ray binary 4U 1820--30 is accreting helium rich material and is
the system most likely to show this unstable mode,especially when it is not
exhibiting X-ray bursts. Our discovery of an unstable mode in a thermally
stable atmosphere shows that nonradial perturbations have a different stability
criterion than the spherically symmetric thermal perturbations that generate
type I X-ray bursts.Comment: Accepted for publication in Astrophysical Journal, 22 pages, 14
figure
INTEGRAL, Swift, and RXTE observations of the 518 Hz accreting transient pulsar Swift J1749.4-2807
The burst-only Swift J1749.4-2807 source was discovered in a high
X-ray-active state, while during an {INTEGRAL observations of the Galactic
bulge on 2010 April 10. Pulsations at 518 Hz were discovered in the RXTE data,
confirming previous suggestions of possible associations between burst-only
sources and accreting millisecond X-ray pulsars. The subsequent discovery of
X-ray eclipses made Swift J1749.42807 the first eclipsing accreting
millisecond X-ray pulsar. We obtain additional information on Swift
J1749.4-2807 and other burst-only sources. We report on the results of a
monitoring campaign on the source, carried out for about two weeks with the
Swift, INTEGRAL, and RXTE satellites.
The observations showed that the X-ray spectrum (energy range 0.5-40 keV) of
Swift J1749.4-2807 during the entire event was accurately modeled by an
absorbed power-law model (N_H~3e2 cm^-2, Gamma~1.7). X-ray eclipses were also
detected in the Swift data and provides a clear evidence of a dust-scattering
halo located along the line of sight to the source. Only one type-I X-ray burst
was observed throughout the two-weeks long monitoring. The X-ray flux of Swift
J1749.4-2807 decayed below the detection threshold of Swift/XRT about 11 days
after the discovery, in a exponential fashion (e-folding time of tau=12^+7_-3
days). We compare the properties of the outburst observed from Swift
J1749.4-2807 with those of the previously known millisecond X-ray pulsars and
other transient low mass X-ray binaries.Comment: Accepted for publication on A&
Unveiling the hard X-ray spectrum from the "burst-only" source SAX J1753.5-2349 in outburst
Discovered in 1996 by BeppoSAX during a single type-I burst event, SAX
J1753.5-2349 was classified as "burst-only" source. Its persistent emission,
either in outburst or in quiescence, had never been observed before October
2008, when SAX J1753.5-2349 was observed for the first time in outburst. Based
on INTEGRAL observations,we present here the first high-energy emission study
(above 10 keV) of a so-called "burst-only". During the outburst the SAX
J1753.5-2349 flux decreased from 10 to 4 mCrab in 18-40 keV, while it was found
being in a constant low/hard spectral state. The broad-band (0.3-100 keV)
averaged spectrum obtained by combining INTEGRAL/IBIS and Swift/XRT data has
been fitted with a thermal Comptonisation model and an electron temperature >24
keV inferred. However, the observed high column density does not allow the
detection of the emission from the neutron star surface. Based on the whole set
of observations of SAX J1753.5-2349, we are able to provide a rough estimate of
the duty cycle of the system and the time-averaged mass-accretion rate. We
conclude that the low to very low luminosity of SAX J1753.5-2349 during
outburst may make it a good candidate to harbor a very compact binary system.Comment: 5 pages, 3 figures, 2 tables; accepted for publication in MNRAS
Letter
Long term flux variations in Cen X-3: clues from flux dependent orbital modulation and pulsed fraction
We have investigated the long term flux variation in Cen X-3 using orbital
modulation and pulsed fraction in different flux states using observations made
with the All Sky Monitor and the Proportional Counter Array on board the Rossi
X-ray Timing Explorer. In the high state, the eclipse ingress and egress are
found to be sharp whereas in the intermediate state the transitions are more
gradual. In the low state, instead of eclipse ingress and egress, the
lightcurve shows a smooth flux variation with orbital phase. The orbital
modulation of the X-ray lightcurve in the low state shows that the X-ray
emission observed in this state is from an extended object. The flux dependent
orbital modulations indicate that the different flux states of Cen X-3 are
primarily due to varying degree of obscuration. Measurement of the pulsed
fraction in different flux states is consistent with the X-ray emission of Cen
X-3 having one highly varying component with a constant pulsed fraction and an
unpulsed component and in the low state, the unpulsed component becomes
dominant. The observed X-ray emission in the low state is likely to be due to
scattering of X-rays from the stellar wind of the companion star. Though we can
not ascertain the origin and nature of the obscuring material that causes the
aperiodic long term flux variation, we point out that a precessing accretion
disk driven by radiative forces is a distinct possibility.Comment: 10 pages, 5 figures. Paper accepted for publication in MNRA
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